This invention relates to a uniaxially oriented polymeric film having an inert filler dispersed therein, a process for preparing such a film, and the use of such film in controlled atmosphere storage containers for fresh fruits and vegetables.
Maintaining the flavor, texture, and eating qualities of fresh fruits and vegetables and extending the shelf life of flowers (hereinafter "produce" collectively) from the time of harvest through the time of consumption is an obvious problem. The most commonly used technique has been refrigeration. Some items, such as tomatoes, bananas, and citrus fruits, are routinely picked in a less-than-ripe condition and stored at reduced temperatures until they are sold. Other products, such as grapes and lettuce, are picked at maturity and refrigerated. The reduced temperature helps to retard further ripening, but only for relatively short time periods and may be detrimental to the keeping quality of the product after it is exposed to room temperature.
For each produce type an optimum range of concentrations of CO.sub.2 and O.sub.2 exists at which its respiration is retarded and quality is improved to the greatest extent. For instance, some produce benefit from relatively high levels of CO.sub.2, e.g., strawberries and mushrooms, while others such as lettuce and tomatoes store better at lower levels of CO.sub.2. Likewise, each produce type also has its own individual respiration rate, which can be expressed as cubic centimeters of oxygen per kg/hour.
It is known that the maturation-rate of produce can be reduced by controlling the atmosphere surrounding the produce so that an optimum O.sub.2 range and relative concentrations of CO.sub.2 to O.sub.2 are maintained. For example, U.S. Pat. No. 4,842,875 discloses a packaging container for controlling the atmosphere during storage of produce to improve retention of the product's freshness by adjusting the CO.sub.2 to O.sub.2 ratio; the environment is controlled by providing a biaxially oriented membrane panel of limited CO.sub.2 and O.sub.2 permeance in an otherwise substantially impermeable container. Other controlled atmosphere packaging containers are disclosed in U.S. Pat. Nos. 3,102,777 and 3,450,542 and publication entitled "Controlling Atmosphere in a Fresh-Fruit Package" by P. Veeraju and M. Karel, Modern Packaging, Vol. 40, #2 (1966), pages 168, 170, 172, 174, and 254.
It is also known that thermoplastic polymers can be filled with inert fillers, cast into sheets, and stretched to form an oriented thermoplastic film. Examples of such methods are disclosed, for example, in U.S. Pat. Nos. 3,903,234, 3,773,608, 4,359,497, and 4,626,252 and British Patent 2,151,538 A. The ingredients and process parameters used determine the characteristics of the resulting product. Hence, a wide range of products have been produced depending on the needs of the industry. In the packaging industry a need still exists for producing a microporous film having highly consistent CO.sub.2 and O.sub.2 permeances throughout the film and a wide processing window.
None of the above mentioned prior art discloses the instant invention.
Note that a more accurate measurement of the ingredients in the composition of the instant invention is by volume rather than by weight because the densities of the ingredients differ significantly. For example, hollow glass beads are extremely low density while CaCO.sub.3 is relatively heavy compared to the hollow glass beads. In the same manner BaSO.sub.4 has yet a relatively different weight per unit volume than both the glass beads and CaCO.sub.3. Other materials having different densities would also vary significantly by weight while their volume requirements would remain relatively close for optimum effectiveness in the instant invention. Hence, while a weight basis percentage range would have to be extremely wide to encompass all of these ingredients, a percentage by volume range would remain narrow. Both measurements are provided throughout this application except for the claims for the sake of comparison.